How do I know if an iphone user has blocked me? More info below?

How do I know if an iphone user has blocked me? More info below?

Added (1). I think my boyfriend blocked me. Whenever I try to send him a text it goes through as blue and the 'delivered' sign shows under it. I had my best friend block me to see if the same thing would happen, since she also has an iphone, but when I would text her the message would send as blue but the 'delivered' sign wouldn't appear under the text. I'm confused

You could try to call the number in question, if it repeatedly goes to voicemail or dial tone after one ring, chances are you're blocked. You can do this without showing your caller ID if you wish.

Other than that, he may just have it in 'Do not disturb mode'.